1. Field of the Invention
The present invention relates to an image forming apparatus which transfers an image recorded in an image recording medium such as microfilm to a recording medium such as paper by means of an electronic photographing method.
2. Description of the Prior Art
Heretofore an image recording medium such as microfilm has been in wide use in recording and storing various kinds of information. There is provided an apparatus which can projects the image on the image recording medium by the use of electronic photographing method and which can record the image on a recording medium such as paper as need arises.
Such a prior example of the apparatus will be described in the following with microfilm reader-printer as an example.
In the microfilm reader-printer, projected light on an image recorded on a microfilm is scanned by a turning mirror. The projected light can be viewed as a projected image or can be utilized to form a latent image on an image carrier (also called a photosensitive drum hereinafter), and the original image is made to reappear on the recording medium by carrying out processings of inverted and direct development, transfer, fixing, and so forth on the latent image.
Now, in a prior apparatus of this kind, when either of the development processings of inverted and direct developments is in progress, the other development means is kept in the on-state while the photosensitive drum is in rotation. The development means consists, for example, of a cylindrical development roller and a magnet which is provided in the inside of the development roller. Such a development means is provided in the neighborhood of the photosensitive drum, and when it is not in use, by keeping it in the state where there is applied a voltage that is to be applied when it is in use, that is, in the on-state, the developer magnetic brush that is formed on the surface of the development roller is removed by displacing the magnet that is housed in its interior.
However, with such a control of the development apparatus which is a development means, it is not possible to remove completely the magnetic brush of the developer off the surface of the development roller. As a result, while one development apparatus is in operation, development by the developer for the other development apparatus which is not in operation will be carried out, so that there was a problem that it becomes a cause of color mixing. Further, in such an image formation apparatus that can carry out inverted an direct developments, each of the two development apparatus develops the portion that was not developed by the other development apparatus. This leads to the so-called "fog phenomenon" which degrades the quality of the image.
Moreover, in a prior microfilm reader-printer, a latent image is formed on an image carrier (photosensitive drum) based on the projected light on an image recorded on the microfilm, develops the latent image on a recording medium in response to a predetermined development mode, and reproduces the original image on a recording medium by carrying out the processings of transfer, fixing, and so on, and the electric charge charged on the recording medium is detached from the image carrier by discharging it by a predetermined voltage that is supplied from a detachment means.
Now, the detachment conditions of the recording medium from the image carrier in such an apparatus, are strongly influenced by the environmental conditions such as the temperature and humidity and the quality of the paper, and so on. Because of this, the width of tolerable variations of the AC voltage that is supplied from the detachment means to the recording medium is often limited to a certain range (for instance, within .+-.3% of the set value).
Further, in an image forming apparatus that has a plurality of development modes, the development method, characteristics of the developer, and others, are often different for each of the mode, so that the detachment conditions are also affected by the difference in modes.
However, in a detachment method of this kind, it has a construction that the AC output voltage of the detachment power supply is held fixed and cannot be changed in response to the development mode. Because of this, there may occur a state in which detachment is impossible in a specific development mode. Or, even when there exists a detachable charged region, the width of the tolerable variations of the AC output voltage may be narrow. Then, the construction of a detachment means which is equipped with a detachment power supply that can satisfy the detachment conditions for all of the development modes will lead to a problem that would result in a sharp increase in the cost of the apparatus.
Moreover, in the case of an image forming apparatus which can carry out both of the direct development and the inverted development, the polarity of the output of the transfer charger for the recording medium is varied in response to the development mode. With such an arrangement, the recording medium, immediately after passing the transfer charger, is charged positively and negatively for the direct and inverted developments, respectively. In that case, in the corona discharge due to alternating current, negative component generally has a more satisfactory discharge efficiency so that the detachment of the recording medium is satisfactory for the case of the direct development, whereas detachment becomes impossible for the inverted development.
Moreover, not only in such a case but also in an apparatus with a plurality of development modes, development method, characteristics of the developer and others differ for different development modes, as mentioned earlier, and the detachment conditions are also different according to the development mode. For these reasons, development method and developer that approximately satisfy the required conditions have to be selected, which used to be a serious burden in caryring out the product development.
In order to solve the above problem, there was proposed in the past a system which has a detachment power supply that is constructed in such a way as to vary the DC output voltage to be superposed on the AC output voltage, in response to the development mode. In that case, however, there occurs a problem that there exists a large possibility of having a detachment error, because of the variation in the detachable region (width of the tolerable variations) due to the AC output voltage that accompanies the change in the DC output voltage.